Communication system

ABSTRACT

Provided is a technology capable of securing communication quality without providing an additional function such as phase correction. A base station device and a communication terminal device when operating as a transmitting device rotate inverse fast Fourier transform (IFFT) output, and copy a last portion of the rotated IFFT output to a head of the rotated IFFT output as a cyclic prefix (CP) to thereby generate a transmission signal so that there is no phase rotation at a head of a demodulation reception window set in a receiving device.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/468,595, filed on Jun. 11, 2019, which is a U.S. National StageApplication under 35 U.S.C. § 371 of International Patent ApplicationPCT/JP2017/045842, filed on Dec. 21, 2021, which is based on and claimspriority to Japanese Patent Application No. 2016-247576, filed on Dec.21, 2017; the entire contents of which are incorporated herein byreference.

TECHNICAL FIELD

The present invention relates to a communication system in which radiocommunication is performed between a communication terminal device suchas a user equipment device and a base station device.

BACKGROUND ART

The 3rd generation partnership project (3GPP), the standard organizationregarding the mobile communication system, is studying communicationsystems referred to as long term evolution (LTE) regarding radiosections and system architecture evolution (SAE) regarding the overallsystem configuration including a core network and a radio accessnetwork, which will be hereinafter collectively referred to as a networkas well (for example, see Non-Patent Documents 1 to 4). Thiscommunication system is also referred to as 3.9 generation (3.9 G)system.

As the access scheme of the LTE, orthogonal frequency divisionmultiplexing (OFDM) is used in a downlink direction and single carrierfrequency division multiple access (SC-FDMA) is used in an uplinkdirection. Further, differently from the wideband code division multipleaccess (W-CDMA), circuit switching is not provided but a packetcommunication system is only provided in the LTE.

The decisions by 3GPP regarding the frame configuration in the LTEsystem described in Non-Patent Document 1 (Chapter 5) will be describedwith reference to FIG. 1. FIG. 1 is a diagram illustrating theconfiguration of a radio frame used in the LTE communication system.With reference to FIG. 1, one radio frame is 10 ms. The radio frame isdivided into ten equally sized subframes. The subframe is divided intotwo equally sized slots. The first and sixth subframes contain adownlink synchronization signal per radio frame. The synchronizationsignals are classified into a primary synchronization signal (P-SS) anda secondary synchronization signal (S-SS).

Non-Patent Document 1 (Chapter 5) describes the decisions by 3GPPregarding the channel configuration in the LTE system. It is assumedthat the same channel configuration is used in a closed subscriber group(CSG) cell as that of a non-CSG cell.

A physical broadcast channel (PBCH) is a channel for downlinktransmission from a base station device (hereinafter may be simplyreferred to as a “base station”) to a communication terminal device(hereinafter may be simply referred to as a “communication terminal”)such as a user equipment device (hereinafter may be simply referred toas a “user equipment”). A BCH transport block is mapped to foursubframes within a 40 ms interval. There is no explicit signalingindicating 40 ms timing.

A physical control format indicator channel (PCFICH) is a channel fordownlink transmission from a base station to a communication terminal.The PCFICH notifies the number of orthogonal frequency divisionmultiplexing (OFDM) symbols used for PDCCHs from the base station to thecommunication terminal. The PCFICH is transmitted per subframe.

A physical downlink control channel (PDCCH) is a channel for downlinktransmission from a base station to a communication terminal. The PDCCHnotifies of the resource allocation information for downlink sharedchannel (DL-SCH) being one of the transport channels described below,resource allocation information for a paging channel (PCH) being one ofthe transport channels described below, and hybrid automatic repeatrequest (HARQ) information related to DL-SCH. The PDCCH carries anuplink scheduling grant. The PDCCH carries acknowledgement(Ack)/negative acknowledgement (Nack) that is a response signal touplink transmission. The PDCCH is referred to as an L1/L2 control signalas well.

A physical downlink shared channel (PDSCH) is a channel for downlinktransmission from a base station to a communication terminal. A downlinkshared channel (DL-SCH) that is a transport channel and a PCH that is atransport channel are mapped to the PDSCH.

A physical multicast channel (PMCH) is a channel for downlinktransmission from a base station to a communication terminal. Amulticast channel (MCH) that is a transport channel is mapped to thePMCH.

A physical uplink control channel (PUCCH) is a channel for uplinktransmission from a communication terminal to a base station. The PUCCHcarries Ack/Nack that is a response signal to downlink transmission. ThePUCCH carries a channel quality indicator (CQI) report. The CQI isquality information indicating the quality of received data or channelquality. In addition, the PUCCH carries a scheduling request (SR).

A physical uplink shared channel (PUSCH) is a channel for uplinktransmission from a communication terminal to a base station. An uplinkshared channel (UL-SCH) that is one of the transport channels is mappedto the PUSCH.

A physical hybrid ARQ indicator channel (PHICH) is a channel fordownlink transmission from a base station to a communication terminal.The PHICH carries Ack/Nack that is a response signal to uplinktransmission. A physical random access channel (PRACH) is a channel foruplink transmission from the communication terminal to the base station.The PRACH carries a random access preamble.

A downlink reference signal (RS) is a known symbol in the LTEcommunication system. The following five types of downlink referencesignals are defined: a cell-specific reference signal (CRS), an MBSFNreference signal, a data demodulation reference signal (DM-RS) being auser equipment-specific reference signal (UE-specific reference signal),a positioning reference signal (PRS), and a channel state informationreference signal (CSI-RS). The physical layer measurement objects of acommunication terminal include reference signal received power (RSRP).

The transport channels described in Non-Patent Document 1 (Chapter 5)will be described. A broadcast channel (BCH) among the downlinktransport channels is broadcast to the entire coverage of a base station(cell). The BCH is mapped to the physical broadcast channel (PBCH).

Retransmission control according to a hybrid ARQ (HARQ) is applied to adownlink shared channel (DL-SCH). The DL-SCH can be broadcast to theentire coverage of the base station (cell). The DL-SCH supports dynamicor semi-static resource allocation. The semi-static resource allocationis also referred to as persistent scheduling. The DL-SCH supportsdiscontinuous reception (DRX) of a communication terminal for enablingthe communication terminal to save power. The DL-SCH is mapped to thephysical downlink shared channel (PDSCH).

The paging channel (PCH) supports DRX of the communication terminal forenabling the communication terminal to save power. The PCH is requiredto be broadcast to the entire coverage of the base station (cell). ThePCH is mapped to physical resources such as the physical downlink sharedchannel (PDSCH) that can be used dynamically for traffic.

The multicast channel (MCH) is used for broadcast to the entire coverageof the base station (cell). The MCH supports SFN combining of multimediabroadcast multicast service (MBMS) services (MTCH and MCCH) inmulti-cell transmission. The MCH supports semi-static resourceallocation. The MCH is mapped to the PMCH.

Retransmission control according to a hybrid ARQ (HARQ) is applied to anuplink shared channel (UL-SCH) among the uplink transport channels. TheUL-SCH supports dynamic or semi-static resource allocation. The UL-SCHis mapped to the physical uplink shared channel (PUSCH).

A random access channel (RACH) is limited to control information. TheRACH involves a collision risk. The RACH is mapped to the physicalrandom access channel (PRACH).

The HARQ will be described. The HARQ is the technique for improving thecommunication quality of a channel by combination of automatic repeatrequest (ARQ) and error correction (forward error correction). The HARQis advantageous in that error correction functions effectively byretransmission even for a channel whose communication quality changes.In particular, it is also possible to achieve further qualityimprovement in retransmission through combination of the receptionresults of the first transmission and the reception results of theretransmission.

An example of the retransmission method will be described. If thereceiver fails to successfully decode the received data, in other words,if a cyclic redundancy check (CRC) error occurs (CRC=NG), the receivertransmits “Nack” to the transmitter. The transmitter that has received“Nack” retransmits the data. If the receiver successfully decodes thereceived data, in other words, if a CRC error does not occur (CRC=OK),the receiver transmits “AcK” to the transmitter. The transmitter thathas received “Ack” transmits the next data.

The logical channels described in Non-Patent Document 1 (Chapter 6) willbe described. A broadcast control channel (BCCH) is a downlink channelfor broadcast system control information. The BCCH that is a logicalchannel is mapped to the broadcast channel (BCH) or downlink sharedchannel (DL-SCH) that is a transport channel.

A paging control channel (PCCH) is a downlink channel for transmittingpaging information and system information change notifications. The PCCHis used when the network does not know the cell location of acommunication terminal. The PCCH that is a logical channel is mapped tothe paging channel (PCH) that is a transport channel.

A common control channel (CCCH) is a channel for transmission controlinformation between communication terminals and a base station. The CCCHis used in the case where the communication terminals have no RRCconnection with the network. In the downlink direction, the CCCH ismapped to the downlink shared channel (DL-SCH) that is a transportchannel. In the uplink direction, the CCCH is mapped to the uplinkshared channel (UL-SCH) that is a transport channel.

A multicast control channel (MCCH) is a downlink channel forpoint-to-multipoint transmission. The MCCH is used for transmission ofMBMS control information for one or several MTCHs from a network to acommunication terminal. The MCCH is used only by a communicationterminal during reception of the MBMS. The MCCH is mapped to themulticast channel (MCH) that is a transport channel.

A dedicated control channel (DCCH) is a channel that transmits dedicatedcontrol information between a communication terminal and a network on apoint-to-point basis. The DCCH is used when the communication terminalhas an RRC connection. The DCCH is mapped to the uplink shared channel(UL-SCH) in uplink and mapped to the downlink shared channel (DL-SCH) indownlink.

A dedicated traffic channel (DTCH) is a point-to-point communicationchannel for transmission of user information to a dedicatedcommunication terminal. The DTCH exists in uplink as well as downlink.The DTCH is mapped to the uplink shared channel (UL-SCH) in uplink andmapped to the downlink shared channel (DL-SCH) in downlink.

A multicast traffic channel (MTCH) is a downlink channel for trafficdata transmission from a network to a communication terminal. The MTCHis a channel used only by a communication terminal during reception ofthe MBMS. The MTCH is mapped to the multicast channel (MCH).

CGI represents a cell global identifier. ECGI represents an E-UTRAN cellglobal identifier. A closed subscriber group (CSG) cell is introduced inthe LTE, and the long term evolution advanced (LTE-A) and universalmobile telecommunication system (UMTS) described below.

The closed subscriber group (CSG) cell is a cell in which subscriberswho are allowed use are specified by an operator (hereinafter, alsoreferred to as a “cell for specific subscribers”). The specifiedsubscribers are allowed to access one or more cells of a public landmobile network (PLMN). One or more cells to which the specifiedsubscribers are allowed access are referred to as “CSG cell(s)”. Notethat access is limited in the PLMN.

The CSG cell is part of the PLMN that broadcasts a specific CSG identity(CSG ID) and broadcasts “TRUE” in a CSG indication. The authorizedmembers of the subscriber group who have registered in advance accessthe CSG cells using the CSG ID that is the access permissioninformation.

The CSG ID is broadcast by the CSG cell or cells. A plurality of CSG IDsexist in the LTE communication system. The CSG IDs are used by userequipments (UEs) for making access from CSG-related members easier.

The locations of communication terminals are tracked based on an areacomposed of one or more cells. The locations are tracked for enablingtracking the locations of communication terminals and callingcommunication terminals, in other words, incoming calling tocommunication terminals even in an idle state. An area for trackinglocations of communication terminals is referred to as a tracking area.

3GPP is studying base stations referred to as Home-NodeB (Home-NB; HNB)and Home-eNodeB (Home-eNB; HeNB). HNB/HeNB is a base station for, forexample, household, corporation, or commercial access service inUTRAN/E-UTRAN. Non-Patent Document 2 discloses three different modes ofthe access to the HeNB and HNB. Specifically, an open access mode, aclosed access mode, and a hybrid access mode are disclosed.

Further, 3GPP is pursuing specifications standard of long term evolutionadvanced (LTE-A) as Release 10 (see Non-Patent Documents 3 and 4). TheLTE-A is based on the LTE radio communication system and is configuredby adding several new techniques to the system.

Carrier aggregation (CA) is studied for the LTE-A system, in which twoor more component carriers (CCs) are aggregated to support widertransmission bandwidths up to 100 MHz. Non-Patent Document 1 describesthe CA.

In the case where CA is configured, a user equipment has a single RRCconnection with a network (NW). In RRC connection, one serving cellprovides NAS mobility information and security input. This cell isreferred to as a primary cell (PCell). In downlink, a carriercorresponding to PCell is a downlink primary component carrier (DL PCC).In uplink, a carrier corresponding to PCell is an uplink primarycomponent carrier (UL PCC).

A secondary cell (SCell) is configured to form a serving cell group witha PCell, in accordance with the user equipment capability. In downlink,a carrier corresponding to SCell is a downlink secondary componentcarrier (DL SCC). In uplink, a carrier corresponding to SCell is anuplink secondary component carrier (UL SCC).

A serving cell group of one PCell and one or more SCells is configuredfor one UE.

The new techniques in the LTE-A include the technique of supportingwider bands (wider bandwidth extension) and the coordinated multiplepoint transmission and reception (CoMP) technique. The CoMP studied forLTE-A in 3GPP is described in Non-Patent Document 1.

The traffic flow of a mobile network is on the rise, and thecommunication rate is also increasing. It is expected that thecommunication rate will be further increased when the operations of theLTE and the LTE-A are fully initiated.

Furthermore, 3GPP is studying the use of small eNBs (hereinafter alsoreferred to as “small-scale base station devices”) configuring smallcells to satisfy tremendous traffic in the future. In an exampletechnique under study, etc., a large number of small eNBs will beinstalled to configure a large number of small cells, thus increasingspectral efficiency and communication capacity. The specific techniquesinclude dual connectivity (abbreviated as DC) in which a user equipmentcommunicates with two eNBs through connection thereto. Non-PatentDocument 1 describes the DC.

Among eNBs that perform dual connectivity (DC), one of them may bereferred to as a master eNB (abbreviated as MeNB), and the other may bereferred to as a secondary eNB (abbreviated as SeNB).

For increasingly sophisticated mobile communications, the fifthgeneration (hereinafter also referred to as “5G”) radio access system isstudied, whose service is aimed to be launched in 2020 and afterward.For example, in the Europe, an organization named METIS summarizes therequirements for 5G (see Non-Patent Document 5).

Among the requirements in the 5G radio access system are a systemcapacity 1000 times as high as, a data transmission rate 100 times ashigh as, a data latency one tenth ( 1/10) as low as, and simultaneouslyconnected communication terminals 100 times as many as those in the LTEsystem, to further reduce the power consumption and device cost.

To satisfy such requirements, increasing the transmission capacity ofdata using broadband frequencies, and increasing the transmission rateof data through increase in the spectral efficiency are being studied.To realize these, the techniques enabling the spatial multiplexing suchas the Multiple Input Multiple Output (MIMO) and the beamforming using amulti-element antenna are being studied.

The MIMO is continuously studied also in LTE-A. From Release 13, fulldimension (FD)-MIMO is studied as the extension of the MIMO, which usestwo-dimensional antenna array. Non-Patent Document 6 describes theFD-MIMO.

It is studied that the 5G radio access system will be installedconcurrently with the LTE system in the initial period of the launch ofits service, which is scheduled in 2020. The following configuration isconsidered. Specifically, an LTE base station and a 5G base station areconnected in a DC configuration, and the LTE base station is regarded asan MeNB and the 5G base station as an SeNB. C-plane data is processed inthe LTE base station having a large cell range, and U-plane is processedin the LTE base station and the 5G base station.

PRIOR ART DOCUMENTS Non-Patent Documents

-   Non-Patent Document 1: 3GPP TS36.300 V13.0.0-   Non-Patent Document 2: 3GPP S1-083461-   Non-Patent Document 3: 3GPP TR 36.814 V9.0.0-   Non-Patent Document 4: 3GPP TR 36.912 V10.0.0-   Non-Patent Document 5: “Scenarios, requirements and KPIs for 5G    mobile and wireless system”, [online], Apr. 30, 2013,    ICT-317669-METIS/D1.1, [Searched on Jan. 25, 2016], Internet    https://www.metis2020.com/documents/deliverables/Non-Patent Document    6: 3GPP TR36.897 V13.0.0

SUMMARY Problem to be Solved by the Invention

Introduction of the 5G system has been studied. Since large-volumecommunication is required, further increase in the bandwidth (the sizeof CC) and further increase in a modulation level than ever have beenstudied, based on OFDM communication. Further, low-latency communicationis also required, and extension of an OFDM sub-carrier interval (OFDMsymbol 15 kHz-+60 kHz×n, or 75 kHz×n) has been studied. Therefore,required accuracy of a phase error has been increased.

Meanwhile, in the conventional OFDM communication, a technology ofcopying a last portion of IFFT output to a head of the OFDM symbol afterIFFT as a cyclic prefix (which may also be hereinafter referred to asCP) is known. According to this, even when a signal corresponding to anamount of one symbol time is taken out of any portion of a CP+OFDMsymbol, fast Fourier transform (FFT), i.e., demodulation, can beperformed. In general, the receiver sets approximately a center of a CPlength as a demodulation start timing (i.e., a reception window head) inconsideration of a control error.

There has hitherto been no problem with the above-mentioned method.However, as the required accuracy of a phase error is increased, thefollowing problem occurs. Specifically, when the reception window headis deviated from the head of the OFDM symbol after IFFT, phase rotationoccurs, and its phase rotation amount depends on frequency. Therefore, adifference of the phase rotation amount in every frequency reducescommunication quality.

The present invention has an object to provide a technology capable ofsecuring communication quality without providing an additional functionsuch as phase correction.

Means to Solve the Problem

A communication system according to the present invention includes abase station device, and a communication terminal device configured toperform radio communication with the base station device. The basestation device and the communication terminal device when operating as atransmitting device rotate inverse fast Fourier transform (IFFT) output,and copy a last portion of the rotated IFFT output to a head of therotated IFFT output as a cyclic prefix (CP) to thereby generate atransmission signal so that there is no phase rotation at a head of ademodulation reception window set in a receiving device.

Effects of the Invention

According to the communication system of the present invention,communication quality can be secured without providing an additionalfunction such as phase correction.

These and other objects, features, aspects and advantages of the presentinvention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating the configuration of a radio frame foruse in an LTE communication system.

FIG. 2 is a block diagram showing the overall configuration of an LTEcommunication system 200 under discussion of 3GPP.

FIG. 3 is a block diagram showing the configuration of a user equipment202 shown in FIG. 2, which is a communication terminal according to thepresent invention.

FIG. 4 is a block diagram showing the configuration of a base station203 shown in FIG. 2, which is a base station according to the presentinvention.

FIG. 5 is a block diagram showing the configuration of an MME accordingto the present invention.

FIG. 6 is a flowchart showing an outline from a cell search to an idlestate operation performed by a user equipment (UE) in the LTEcommunication system.

FIG. 7 is a diagram illustrating occurrence of phase rotation at a headof a demodulation reception window in OFDM.

FIG. 8 is a diagram illustrating rotating IFFT output to match phases inevery frequency at the head of the demodulation reception windowaccording to a first embodiment.

FIG. 9 is a sequence diagram illustrating an example of controlling arotation amount of the IFFT output by using an RRC message according tothe first embodiment.

FIG. 10 is a sequence diagram illustrating an example of controlling therotation amount of the IFFT output by using a control signalaccompanying with a message such as PDCCH according to the firstembodiment.

FIG. 11 is a diagram illustrating controlling of the demodulationreception window in consideration of multipath spread under a state of amultipath according to the first embodiment.

FIG. 12 is a diagram illustrating controlling of the demodulationreception window in consideration of weighted multipath spread under astate of a multipath according to the first embodiment.

FIG. 13 is a diagram illustrating controlling of the demodulationreception window in consideration of delay time of a maximum path undera state of a multipath according to the first embodiment.

FIG. 14 is a diagram illustrating setting of approximately 1/2 of theshortest CP length to the head of the demodulation reception windowaccording to a second embodiment.

FIG. 15 is a diagram illustrating setting of the head of thedemodulation reception window in a case of {shortest CP lengthCPmin}>{corresponding cell radius}/{speed of radio wave}/2 according tothe second embodiment.

FIG. 16 is a diagram illustrating setting of the head of thedemodulation reception window in a case of {shortest CP lengthCPmin}<{corresponding cell radius}/{speed of radio wave}/2 according tothe second embodiment.

FIG. 17 is a sequence diagram illustrating a procedure for setting thehead of the demodulation reception window according to the secondembodiment.

FIG. 18 is a diagram for illustrating matching of head positions of thedemodulation reception windows of all of the user equipments accordingto the second embodiment.

FIG. 19 is a diagram illustrating matching of the head positions of thedemodulation reception windows for a reference signal such as DMRS andcontrol information such as PUCCH according to the second embodiment.

FIG. 20 is a sequence diagram illustrating an example in which the userequipment performs communication without SR/SG according to the secondembodiment.

FIG. 21 is a diagram illustrating setting of a position of stoppingdownlink transmission in consideration of a channel assigned at a centerof CC according to a third embodiment.

FIG. 22 is a diagram illustrating an example of transmitting SRS in anon-transmission period of FIG. 21 according to the third embodiment.

FIG. 23 is a diagram illustrating an example of setting SRS to 1/2 of anOFDM symbol length assigned at a center frequency of CC according to thethird embodiment.

DESCRIPTION OF EMBODIMENTS First Embodiment

FIG. 2 is a block diagram showing an overall configuration of an LTEcommunication system 200, which is under discussion of 3GPP. FIG. 2 willbe described. A radio access network is referred to as an evolveduniversal terrestrial radio access network (E-UTRAN) 201. A userequipment device (hereinafter, referred to as a “user equipment (UE)”)202 that is a communication terminal device is capable of radiocommunication with a base station device (hereinafter, referred to as a“base station (E-UTRAN Node B: eNB)”) 203 and transmits and receivessignals through radio communication.

Here, the “communication terminal device” covers not only a userequipment device such as a movable mobile phone terminal device, butalso an unmovable device such as a sensor. In the following description,the “communication terminal device” may be simply referred to as a“communication terminal”.

The E-UTRAN is composed of one or a plurality of base stations 203,provided that a control protocol for the user equipment 202 such as aradio resource control (RRC), and user planes such as a packet dataconvergence protocol (PDCP), radio link control (RLC), medium accesscontrol (MAC), or physical layer (PHY) are terminated in the basestation 203.

The control protocol radio resource control (RRC) between the userequipment 202 and the base station 203 performs broadcast, paging, RRCconnection management, and the like. The states of the base station 203and the user equipment 202 in RRC are classified into RRC_IDLE andRRC_CONNECTED.

In RRC_IDLE, public land mobile network (PLMN) selection, systeminformation (SI) broadcast, paging, cell re-selection, mobility, and thelike are performed. In RRC_CONNECTED, the user equipment has RRCconnection and is capable of transmitting and receiving data to and froma network. In RRC_CONNECTED, for example, handover (HO) and measurementof a neighbor cell are performed.

The base stations 203 are classified into eNBs 207 and Home-eNBs 206.The communication system 200 includes an eNB group 203-1 including aplurality of eNBs 207 and a Home-eNB group 203-2 including a pluralityof Home-eNBs 206. A system, composed of an evolved packet core (EPC)being a core network and an E-UTRAN 201 being a radio access network, isreferred to as an evolved packet system (EPS). The EPC being a corenetwork and the E-UTRAN 201 being a radio access network may becollectively referred to as a “network”.

The eNB 207 is connected to an MME/S-GW unit (hereinafter, also referredto as an “MME unit”) 204 including a mobility management entity (MME), aserving gateway (S-GW), or an MME and an S-GW by means of an S1interface, and control information is communicated between the eNB 207and the MME unit 204. A plurality of MME units 204 may be connected toone eNB 207. The eNBs 207 are connected to each other by means of an X2interface, and control information is communicated between the eNBs 207.

The Home-eNB 206 is connected to the MME unit 204 by means of an S1interface, and control information is communicated between the Home-eNB206 and the MME unit 204. A plurality of Home-eNBs 206 are connected toone MME unit 204. Or, the Home-eNBs 206 are connected to the MME units204 through a Home-eNB gateway (HeNBGW) 205. The Home-eNB 206 isconnected to the HeNBGW 205 by means of an S1 interface, and the HeNBGW205 is connected to the MME unit 204 by means of an S1 interface.

One or a plurality of Home-eNBs 206 are connected to one HeNBGW 205, andinformation is communicated therebetween through an S1 interface. TheHeNBGW 205 is connected to one or a plurality of MME units 204, andinformation is communicated therebetween through an S1 interface.

The MME units 204 and HeNBGW 205 are entities of higher layer,specifically, higher nodes, and control the connections between the userequipment (UE) 202 and the eNB 207 and the Home-eNB 206 being basestations. The MME units 204 configure an EPC being a core network. Thebase station 203 and the HeNBGW 205 configure the E-UTRAN 201.

Further, 3GPP is studying the configuration below. The X2 interfacebetween the Home-eNBs 206 is supported. In other words, the Home-eNBs206 are connected to each other by means of an X2 interface, and controlinformation is communicated between the Home-eNBs 206. The HeNBGW 205appears to the MME unit 204 as the Home-eNB 206. The HeNBGW 205 appearsto the Home-eNB 206 as the MME unit 204.

The interfaces between the Home-eNBs 206 and the MME units 204 are thesame, which are the S1 interfaces, in both cases where the Home-eNB 206is connected to the MME unit 204 through the HeNBGW 205 and it isdirectly connected to the MME unit 204.

The base station 203 may configure a single cell or a plurality ofcells. Each cell has a range predetermined as a coverage in which thecell can communicate with the user equipment 202 and performs radiocommunication with the user equipment 202 within the coverage. In thecase where one base station 203 configures a plurality of cells, everycell is configured so as to communicate with the user equipment 202.

FIG. 3 is a block diagram showing the configuration of the userequipment 202 of FIG. 2 that is a communication terminal according tothe present invention. The transmission process of the user equipment202 shown in FIG. 3 will be described. First, a transmission data bufferunit 303 stores the control data from a protocol processing unit 301 andthe user data from an application unit 302. The data stored in thetransmission data buffer unit 303 is passed to an encoding unit 304 andis subjected to an encoding process such as error correction. There mayexist the data output from the transmission data buffer unit 303directly to a modulating unit 305 without the encoding process. The dataencoded by the encoding unit 304 is modulated by the modulating unit305. The modulated data is converted into a baseband signal, and thebaseband signal is output to a frequency converting unit 306 and is thenconverted into a radio transmission frequency. After that, atransmission signal is transmitted from an antenna 307 to the basestation 203.

The user equipment 202 executes the reception process as follows. Theradio signal from the base station 203 is received through the antenna307. The received signal is converted from a radio reception frequencyinto a baseband signal by the frequency converting unit 306 and is thendemodulated by a demodulating unit 308. The demodulated data is passedto a decoding unit 309 and is subjected to a decoding process such aserror correction. Among the pieces of decoded data, the control data ispassed to the protocol processing unit 301, and the user data is passedto the application unit 302.

A series of processes by the user equipment 202 is controlled by acontrol unit 310. This means that, though not shown in FIG. 3, thecontrol unit 310 is connected to the individual units 301 to 309.

FIG. 4 is a block diagram showing the configuration of the base station203 of FIG. 2 that is a base station according to the present invention.The transmission process of the base station 203 shown in FIG. 4 will bedescribed. An EPC communication unit 401 performs data transmission andreception between the base station 203 and the EPC (such as the MME unit204), HeNBGW 205, and the like. A communication with another basestation unit 402 performs data transmission and reception to and fromanother base station. The EPC communication unit 401 and thecommunication with another base station unit 402 each transmit andreceive information to and from a protocol processing unit 403. Thecontrol data from the protocol processing unit 403, and the user dataand the control data from the EPC communication unit 401 and thecommunication with another base station unit 402 are stored in atransmission data buffer unit 404.

The data stored in the transmission data buffer unit 404 is passed to anencoding unit 405 and is then subjected to an encoding process such aserror correction. There may exist the data output from the transmissiondata buffer unit 404 directly to a modulating unit 406 without theencoding process. The encoded data is modulated by the modulating unit406. The modulated data is converted into a baseband signal, and thebaseband signal is output to a frequency converting unit 407 and is thenconverted into a radio transmission frequency. After that, atransmission signal is transmitted from an antenna 408 to one or aplurality of user equipments 202.

The reception process of the base station 203 is executed as follows. Aradio signal from one or a plurality of user equipments 202 is receivedthrough the antenna 408. The received signal is converted from a radioreception frequency into a baseband signal by the frequency convertingunit 407, and is then demodulated by a demodulating unit 409. Thedemodulated data is passed to a decoding unit 410 and is then subjectedto a decoding process such as error correction. Among the pieces ofdecoded data, the control data is passed to the protocol processing unit403, the EPC communication unit 401, or the communication with anotherbase station unit 402, and the user data is passed to the EPCcommunication unit 401 and the communication with another base stationunit 402.

A series of processes by the base station 203 is controlled by a controlunit 411. This means that, though not shown in FIG. 4, the control unit411 is connected to the individual units 401 to 410.

FIG. 5 is a block diagram showing the configuration of the MME accordingto the present invention. FIG. 5 shows the configuration of an MME 204 aincluded in the MME unit 204 shown in FIG. 2 described above. A PDN GWcommunication unit 501 performs data transmission and reception betweenthe MME 204 a and the PDN GW. A base station communication unit 502performs data transmission and reception between the MME 204 a and thebase station 203 by means of the S1 interface. In the case where thedata received from the PDN GW is user data, the user data is passed fromthe PDN GW communication unit 501 to the base station communication unit502 via a user plane communication unit 503 and is then transmitted toone or a plurality of base stations 203. In the case where the datareceived from the base station 203 is user data, the user data is passedfrom the base station communication unit 502 to the PDN GW communicationunit 501 via the user plane communication unit 503 and is thentransmitted to the PDN GW.

In the case where the data received from the PDN GW is control data, thecontrol data is passed from the PDN GW communication unit 501 to acontrol plane control unit 505. In the case where the data received fromthe base station 203 is control data, the control data is passed fromthe base station communication unit 502 to the control plane controlunit 505.

A HeNBGW communication unit 504 is provided in the case where the HeNBGW205 is provided, which performs data transmission and reception betweenthe MME 204 a and the HeNBGW 205 by means of the interface (IF)according to an information type. The control data received from theHeNBGW communication unit 504 is passed from the HeNBGW communicationunit 504 to the control plane control unit 505. The processing resultsof the control plane control unit 505 are transmitted to the PDN GW viathe PDN GW communication unit 501. The processing results of the controlplane control unit 505 are transmitted to one or a plurality of basestations 203 by means of the S1 interface via the base stationcommunication unit 502, and are transmitted to one or a plurality ofHeNBGWs 205 via the HeNBGW communication unit 504.

The control plane control unit 505 includes a NAS security unit 505-1,an SAE bearer control unit 505-2, and an idle state mobility managingunit 505-3, and performs an overall process for the control plane. TheNAS security unit 505-1 provides, for example, security of a non-accessstratum (NAS) message. The SAE bearer control unit 505-2 manages, forexample, a system architecture evolution (SAE) bearer. The idle statemobility managing unit 505-3 performs, for example, mobility managementof an idle state (LTE-IDLE state, which is merely referred to as idle aswell), generation and control of a paging signal in the idle state,addition, deletion, update, and search of a tracking area of one or aplurality of user equipments 202 being served thereby, and tracking arealist management.

The MME 204 a distributes a paging signal to one or a plurality of basestations 203. In addition, the MME 204 a performs mobility control of anidle state. When the user equipment is in the idle state and an activestate, the MME 204 a manages a list of tracking areas. The MME 204 abegins a paging protocol by transmitting a paging message to the cellbelonging to a tracking area in which the UE is registered. The idlestate mobility managing unit 505-3 may manage the CSG of the Home-eNBs206 to be connected to the MME 204 a, CSG IDs, and a whitelist.

An example of a cell search method in a mobile communication system willbe described next. FIG. 6 is a flowchart showing an outline from a cellsearch to an idle state operation performed by a communication terminal(UE) in the LTE communication system. When starting a cell search, inStep ST601, the communication terminal synchronizes slot timing andframe timing by a primary synchronization signal (P-SS) and a secondarysynchronization signal (S-SS) transmitted from a neighbor base station.

The P-SS and S-SS are collectively referred to as a synchronizationsignal (SS). Synchronization codes, which correspond one-to-one to PCIsassigned per cell, are assigned to the synchronization signals (SSs).The number of PCIs is currently studied in 504 ways. The 504 ways ofPCIs are used for synchronization, and the PCIs of the synchronizedcells are detected (specified).

In Step ST602, next, the user equipment detects a cell-specificreference signal (CRS) being a reference signal (RS) transmitted fromthe base station per cell and measures the reference signal receivedpower (RSRP). The codes corresponding one-to-one to the PCIs are usedfor the reference signal RS. Separation from another cell is enabled bycorrelation using the code. The code for RS of the cell is derived fromthe PCI specified in Step ST601, so that the RS can be detected and theRS received power can be measured.

In Step ST603, next, the user equipment selects the cell having the bestRS received quality, for example, the cell having the highest RSreceived power, that is, the best cell, from one or more cells that havebeen detected up to Step ST602.

In Step ST604, next, the user equipment receives the PBCH of the bestcell and obtains the BCCH that is the broadcast information. A masterinformation block (MIB) containing the cell configuration information ismapped to the BCCH over the PBCH. Accordingly, the MIB is obtained byobtaining the BCCH through reception of the PBCH. Examples of the MIBinformation include the downlink (DL) system bandwidth (also referred toas a transmission bandwidth configuration (dl-bandwidth)), the number oftransmission antennas, and a system frame number (SFN).

In Step ST605, next, the user equipment receives the DL-SCH of the cellbased on the cell configuration information of the MIB, to therebyobtain a system information block (SIB) 1 of the broadcast informationBCCH. The SIB1 contains the information about the access to the cell,information about cell selection, and scheduling information on anotherSIB (SIBk; k is an integer equal to or greater than two). In addition,the SIB1 contains a tracking area code (TAC).

In Step ST606, next, the communication terminal compares the TAC of theSIB1 received in Step ST605 with the TAC portion of a tracking areaidentity (TAI) in the tracking area list that has already been possessedby the communication terminal. The tracking area list is also referredto as a TAI list. TAI is the identification information for identifyingtracking areas and is composed of a mobile country code (MCC), a mobilenetwork code (MNC), and a tracking area code (TAC). MCC is a countrycode. MNC is a network code. TAC is the code number of a tracking area.

If the result of the comparison of Step ST606 shows that the TACreceived in Step ST605 is identical to the TAC included in the trackingarea list, the user equipment enters an idle state operation in thecell. If the comparison shows that the TAC received in Step ST605 is notincluded in the tracking area list, the communication terminal requiresa core network (EPC) including MME and the like to change a trackingarea through the cell for performing tracking area update (TAU).

The device configuring a core network (hereinafter, also referred to asa “core-network-side device”) updates the tracking area list based on anidentification number (such as UE-ID) of a communication terminaltransmitted from the communication terminal together with a TAU requestsignal. The core-network-side device transmits the updated tracking arealist to the communication terminal. The communication terminal rewrites(updates) the TAC list of the communication terminal based on thereceived tracking area list. After that, the communication terminalenters the idle state operation in the cell.

An OFDM transmission method depending on a communication state, e.g.,control of a head position of a reception window, will be describedbelow.

A signal subjected to error correction processing in the encoding unitand rate matching processing for making the data number fit into a radioframe becomes an orthogonal frequency division multiplexing (OFDM)symbol in the modulating unit through modulation processing such as64-QAM and 256-QAM and inverse fast Fourier transform (IFFT). Asillustrated in FIG. 7, a technology of copying a last portion of IFFToutput to a head of the OFDM symbol after IFFT as a cyclic prefix (whichmay also be hereinafter referred to as CP) is known. According to this,even when a signal corresponding to an amount of one symbol time istaken out of any portion of a CP+OFDM symbol, fast Fourier transform(FFT), i.e., demodulation, can be performed. Therefore, communicationcan be performed without causing an error of timing control of ademodulator of a receiver, an error of reception arrival time due to adistance between the user equipment and the base station, andinterference between symbols due to delay spread of a multipath when themultipath occurs. In general, in order to avoid the above-mentionedtiming error and interference between symbols, the receiver setsapproximately a center of a CP length as a demodulation start timing(i.e., a reception window head), and performs timing advance (TA)control.

Here, signals after IFFT output from the same IFFT function unit havephases matching in every frequency at the head of data, but phaserotation occurs when the reception window head is deviated from the headof the data. Especially, as the frequency is higher, the phase rotationamount is larger at the same time. In view of this, in 5G, in order toachieve low-latency communication (data processing delay of 1/10 orless), increase in an OFDM symbol rate has been studied.

However, when a data size of a block (a resource block (RB) ofLTE/LTE-A) specified by frequency and time is the same, a frequencybandwidth of the resource block is large, and frequency deviation in theresource block is also large. For example, when the OFDM symbol rate ischanged from 15 kHz to 240 kHz, phase deviation multiplied by 240/15=16is generated. Further, when 64-QAM and 256-QAM, which have a higherdegree of modulation than QPSK, are used in order to achievelarge-volume communication, it is known that a tolerable phase error isreduced.

From the above, there is a problem in that communication quality isreduced unless the receiver is provided with an additional function ofaccurately correcting the frequency deviation.

Further, when the OFDM symbol rate is increased in order to achievelow-latency communication (data processing delay of 1/10 or less), thereis a problem in that overhead of CP is relatively large with respect tothroughput.

<1-1>

In view of this, the first embodiment provides a technology for solvingthe problems as described above. Specifically, a technology of rotatingan OFDM symbol (i.e., IFFT output) in advance to eliminate phaserotation in the reception window head in the receiver to thereby becapable of securing communication quality without providing anadditional function such as phase correction is provided.

Description will be given with reference to FIG. 8. The modulating unitperforms modulation processing such as 64-QAM and 256-QAM and IFFT onthe signal subjected to the error correction processing in the encodingunit and the rate matching processing for making the data number fitinto a radio frame.

Then, the modulating unit rotates the IFFT output with the followingfirst or second method. In the first method, a predetermined amount onthe head side is rotated to an end side in the IFFT output to betransmitted at certain time. More specifically, when the IFFT output tobe transmitted at certain time has 2048 points of IFFT(0) to IFFT(2047),rotation of IFFT(0)→IFFT(2047)→IFFT(2046)→ . . . →IFFT(1)→IFFT(0) isrepeated a required number of times. In the second method, an inputsignal of IFFT is multiplied by exp(2πt/2048). Here, t is equal to thenumber of times of the above-mentioned repetition in the first method.In other words, the receiver rotates phase of a frequency component atthe time of performing FFT from the reception window head.

After rotation is performed with the first or second method, a lastportion of the OFDM symbol is copied as in the conventional technologyto provide a CP. According to the above-mentioned rotation, when thereceiver performs FFT demodulation, phases in every frequency at a headof a demodulation reception window match.

More specific examples of the rotation of the IFFT output will bedescribed below.

(1) An amount of rotation of the IFFT output (i.e., the number of timesof repetition) is determined as follows.

(1a) A rotation amount of the IFFT output is stored in advance as asystem parameter in non-volatile memory of the base station. At the timeof starting the system, the rotation amount is read out from thenon-volatile memory, and the IFFT output is rotated based on the readout rotation amount.

In this case, when the base station notifies the user equipment of therotation amount of the IFFT output as a system parameter throughbroadcast information, the parameter can be changed at required timesuch as when a corresponding cell radius of the base station is changed,and thus this is more effective. The user equipment adjusts the positionof the reception window by an amount corresponding to the rotationamount obtained from the base station, and then performs demodulation.

Further, when the user equipment performs transmission to the basestation, the user equipment rotates the IFFT output by using theabove-mentioned rotation amount obtained from the base station, and thenperforms transmission. Note that the rotation amount at the time oftransmission from the base station and the rotation amount at the timeof transmission from the user equipment may be equal to each other.

(1b) Although the above-mentioned (1a) illustrates an example in whichthe rotation amount of the IFFT output is stored as a system parameterin the non-volatile memory of the base station, the rotation amount maybe specified by a message from a higher-layer device of the basestation, such as an operations, administration, and management (OAM)server. According to this, when the parameter is changed, the parametercentrally managed in the OAM server may be changed. Specifically, theparameter need not be individually changed in all of the base stations.

(1c) Although the above-mentioned (1b) illustrates an example in whichthe rotation amount of the IFFT output is set by the OAM server, settingthe rotation amount of the IFFT output by using a radio resource control(RRC) message is also effective. Here, for example, the RRC messagecorresponds to RRC Connection Reconfig transmitted from the base stationto the user equipment to correspond to a Bearer Setup request from MME,or corresponds to RRC Connection Reconfig with which a handover commandis transmitted at the time of handover. With use of the RCC message, therotation amount of the IFFT output can be finely set for each userequipment.

Further detailed description will be given with reference to FIG. 9.When the user equipment performs random access in Step ST901, the basestation that has received the random access determines the rotationamount of the IFFT output for each user equipment in Step ST902.Specifically, when a demodulation reception head is set to a positiondeviated from a reference timing by A[ns] in the TA control, therotation amount is determined to be an amount corresponding to {CPlength of radio format to which IFFT output rotation isapplied}−{A[ns]}.

In Step ST903, the base station notifies the user equipment of a resultof the determination of the rotation amount. In Step ST904, the userequipment that has received the result notification transmits a signalcorresponding to RRC Connection Reconfig Complete to the base station.After the base station confirms that the base station has received thesignal corresponding to RRC Connection Reconfig Complete from the userequipment, the base station notifies the user equipment of the rotationamount of the IFFT output through the following communication.

Here, when the rotation amount of the IFFT output is recognizeddifferently between the user equipment and the base station,communication quality is reduced. However, when communication can besecured even with such reduced communication quality, the rotation ofthe IFFT output may be used from Step ST903.

As can be understood from the description above, similar effects can beachieved when a “difference between the demodulation reception head andthe reference timing in the TA control” is specified, instead ofspecifying “the rotation amount of the IFFT output.” Therefore, “therotation amount of the IFFT output” or a “target amount of thedifference between the demodulation reception head and the referencetiming in the TA control” is notified along with the TA control, the TAcontrol and the control of rotating the IFFT output can besimultaneously processed, and processing is facilitated when eachcontrol amount is changed.

(1d) Although the above-mentioned (1c) illustrates an example in whichthe rotation amount of the IFFT output is set by using an RRC message,it is also effective to set by using a control signal of L1 accompanyingwith a message, such as a physical downlink control channel (PDCCH) anda physical uplink control channel (PUCCH). According to this, therotation amount of the IFFT output can be changed for each unit such asa subframe and a slot, i.e., for each unit of data for transmittingPDCCH and PUCCH, and thus finer control is enabled.

FIG. 10 illustrates an example in which the TA control is performed at alow speed, and the demodulation reception window set by the TA controlis actually transmitted from the user equipment. In a case where thedemodulation reception window is deviated from the head of CP of thetransmission data, when the user equipment transmits a known sequence,such as a demodulation reference signal (DMRS) transmitted together withPUSCH, in Step ST911, the base station detects the deviation of arrivaltime from the known sequence in Step ST912. Then, in Step ST913, thebase station notifies the user equipment of correct information of therotation amount of the IFFT output by using PDCCH. In Step ST913, therotation amount for the IFFT output of every case may be notified to theuser equipment. Alternatively, a deviation amount (a rotation amount+Δ,or −Δ) with respect to the rotation amount set at the time of the TAcontrol (i.e., {CP length of radio format to which IFFT output rotationis applied}−{A[ns]}) may be notified. According to this, informationbits can be reduced. The user equipment performs demodulation of PDSCHin accordance with the PDCCH of Step ST913.

<1-2>

Another technology for solving the problems described above is providedbelow. Specifically, a technology of rotating an OFDM symbol (i.e., IFFToutput) in advance to eliminate phase rotation in the reception windowhead in the receiver to thereby be capable of securing communicationquality without providing an additional function such as phasecorrection is provided. Further, a technology of dynamically changingthe CP length depending on a state of a radio channel between the basestation and the user equipment or depending on a combination of the userequipments instantly handled by the base station to thereby enhancethroughput not only by eliminating phase rotation in the receptionwindow head but also by reducing overhead is provided.

(2) A method of managing and controlling the CP length, not managing andcontrolling the rotation amount of the IFFT output will be described.When the rotation amount of the IFFT output is ½ of the CP length, amargin of the reception window can be equally secured when the userequipment and the base station either approach each other or move awayfrom each other. In view of this, an example of managing and controllingon the assumption that a difference between a reference timing and thedemodulation reception window is ½ of the CP length is illustrated. Thisis particularly effective when TA is not performed as in a downlinksignal.

(2a) As the CP length, a value stored in the non-volatile memory of thebase station as a system parameter is used. Note that the CP length maybe specified by the OAM server, or may be specified by an RRC message(e.g., a signal corresponding to RRC Connection Reconfig). The basestation rotates the IFFT output by an amount corresponding to time thatis ½ of the CP length at the time of transmission processing.

In this case, it is more effective when the base station notifies theuser equipment of the CP length as a system parameter through broadcastinformation. The user equipment adjusts the position of the receptionwindow by an amount corresponding to time that is ½ of the CP lengthobtained from the base station, and then performs demodulation.

Further, when the user equipment performs transmission to the basestation, the user equipment rotates the IFFT output by time that is ½ ofthe above-mentioned CP length obtained from the base station, and thenperforms transmission. Note that the rotation amount at the time oftransmission from the base station and the rotation amount at the timeof transmission from the user equipment may be equal to each other.

(2b) Although the above-mentioned (2a) illustrates an example in whichthe CP length is directly specified, the CP length may be specified byan index value (an indicator) that can associate various radio formatsgenerated due to a difference between the OFDM symbol rates or the like.Because the CP length is individually defined in various radio formats,the above-mentioned index value is set such that the same CP length canbe derived in both the base station and the user equipment. The IFFToutput is rotated by time that is ½ of the CP length obtained from theindex value.

In the description above, the rotation amount of the IFFT output is setto ½ of the CP length. This setting is achieved mainly in view of achange margin of a data transmission timing generated due to obscurityas to in which direction among an approaching direction and amoving-away direction the user equipment is to move. Therefore, therotation amount of the IFFT output need not be exact ½ of the CP length,and similar effects can also be achieved with a value around exact ½ ofthe CP length. Specifically, it suffices that the rotation amount of theIFFT output be a value within an allowable range including exact ½ ofthe CP length, i.e., a value substantially equal to ½ of the CP length.

(2c) Although the above-mentioned (2a) illustrates an example in whichthe CP length is set by using an RRC message, it is also effective toset by using a control signal of L1 accompanying with a message, such asPDCCH and PUCCH. According to this, the CP length can be changed foreach unit of data to be transmitted, and thus finer control is enabled.

For example, the CP length itself is reduced when successivetransmission is performed for the same user equipment, and the CP lengthitself is increased when successive transmission is not performed. Inthis manner, overhead can be reduced. Also in this example, the IFFToutput can be rotated in accordance with the change of the CP length.

(3) Although the above-mentioned (2) illustrates an example in which therotation amount of the IFFT output is ½ of the CP length, it is moreeffective to adjust the CP length in consideration of the distancebetween the user equipment and the base station, and set the rotationamount of the IFFT output to ½ of the adjusted CP length. A sequence ofsuch a case is basically the same as FIG. 9 and FIG. 10 described above,and thus will be described with reference to FIG. 9 and FIG. 10.

When the base station receives a signal from the user equipment, thebase station calculates delay time of a reception signal by using aknown sequence in the reception signal, and calculates the distancebetween the user equipment and the base station based on the obtaineddelay time (Steps ST902 and ST912). Here, for example, the knownsequence refers to a preamble of random access transmitted from the userequipment in Step ST901, or DMRS transmitted together with PUSCH fromthe user equipment in Step ST911. After that, when the base stationconfirms a response from the user equipment in Steps ST904 and ST914,the base station applies the rotation amount of the IFFT output. Notethat the same sequence is applied in (4) to (6) described below.

(3a) The distance between the user equipment and the base station istaken into considered with reference to the following formula, forexample. Specifically, {reception window head in stationarystate}+{distance between user equipment and base station}/{speed ofradio wave}={CP length to be set}.

Here, the reception window head in a stationary state is equal to thatof the above-mentioned (2), and is approximately ½ of the CP length, forexample. The distance between the user equipment and the base station islonger as the user equipment further moves away from the base station,which increases the reception window head to be larger than ½ of the CPlength. Consequently, even when a signal from the user equipment arriveslate, phases in every frequency at the head of the demodulationreception window match.

(3b) The distance between the user equipment and the base station may bemeasured by using the global positioning system (GPS). Also in thiscase, the rotation amount of the IFFT output is set similarly to theabove-mentioned (3a).

(4) Although the above-mentioned (2) illustrates an example in which therotation amount of the IFFT output is ½ of the CP length, it is moreeffective to adjust the CP length in consideration of a moving speed anda moving direction of the user equipment, and set the rotation amount ofthe IFFT output to ½ of the adjusted CP length.

(4a) The moving speed and the moving direction of the user equipment aretaken into considered with reference to the following formula, forexample.

Specifically,

{reception window head in stationary state}+{controllable cycle such asRRC/L1}×{speed at which user equipment moves away from basestation}/{speed of radio wave}={CP length to be set}.

Here, the reception window head in a stationary state is equal to thatof the above-mentioned (2), and is approximately ½ of the CP length, forexample. Further, the controllable cycle is approximately average timerequired to transmit an RRC message when the rotation amount of the IFFToutput is controlled by RRC, and substantially has the same order ofmagnitude of approximately 100 ms. When the rotation amount of the IFFToutput is controlled by L1 control, depending on the radio format, thecontrollable cycle is average time required for retransmission of HARQfor controlling L1 the fastest, and substantially has the same order ofmagnitude of approximately 1 to 10 ms.

It is known that the speed at which the user equipment moves away fromthe base station can be calculated from a frequency of a receptioncarrier wave and a phase rotation speed of a known signal, for example(Doppler frequency). Specifically, the speed at which the user equipmentmoves away from the base station can be calculated from the followingformula.

2π×{phase rotation amount in unit time}×{speed of radio wave}/{frequencyof reception carrier wave}.

According to this (4a), the reception window head is increased to belarger than ½ of the CP length. Consequently, even when the userequipment moves away from the base station and a signal from the userequipment arrives late, phases in every frequency at the head of thedemodulation reception window match.

It is more effective when this (4a) is applied to the above-mentioned(3) at the same time.

(4b) The moving speed and the moving direction of the user equipment maybe measured by using GPS. Also in this case, the rotation amount of theIFFT output is set similarly to the above-mentioned (4a).

(5) Although the above-mentioned (2) to (4) illustrate an example inwhich the head position of the demodulation reception window isapproximately ½ of the CP length, the demodulation reception window maybe determined in consideration of delay distribution due to a multipath.Here, reception in the user equipment will be described.

(5a) When an obstruction or a reflecting object exists between the basestation and the user equipment, communication between the base stationand the user equipment is communication with a plurality of paths (i.e.,a multipath), which is different from communication with only one pathin a free space. A state of the multipath is illustrated in FIG. 11.

The multipath is an effective signal, and thus when arrival time of allsignals from a signal arriving early to a signal arriving late fallswithin the range of the CP length, a signal-to-noise-ratio (SN ratio) isimproved. Therefore, it is effective to deviate the head of thedemodulation reception window by an amount corresponding to ½ ofmultipath spread from ½ of the CP length in a forward direction. In thiscase, the rotation amount of the IFFT output is increased by an amountcorresponding to ½ of the multipath spread. Consequently, phases inevery frequency at the head of the demodulation reception window match.

The above-mentioned (5b) illustrates an example in which thedemodulation reception window is controlled to be deviated by an amountcorresponding to ½ of the multipath spread in the forward direction. Incontrast, it is also effective to employ a method of deviating the headof the demodulation reception window by an amount corresponding to delayspread weighted by delay distribution or a reception level from ½ of theCP length in the forward direction, and rotating a larger amount of theIFFT output in accordance with this deviation amount (see FIG. 12).According to this method, in an environment having large path spread,phases in every frequency at the head of the demodulation receptionwindow match on average. An example for explaining the weighted delayspread is illustrated in Table 1.

TABLE 1 Delay time (ns) Electric power (dB) Electric power (true value)0 0 0 200 −0.9 0.813 800 −4.9 0.324 1200 −8 0.158 2300 −7.8 0.166 3700−23.9 0.004

According to the example of Table 1, the weighted delay spread iscalculated as below.

0×0+200×0.813+800×0.324+1200×0.158+2300×0.166+3700×0.004=409 ns

(5c) The above-mentioned example illustrates an example in which thedemodulation reception window is controlled to be deviated by an amountcorresponding to ½ of the multipath spread in the forward direction. Incontrast, it is also effective to employ a method of controlling thedemodulation reception window such that the delay time of a maximum pathis at the position of ½ of the CP length when communication is performedin an area mainly having an open environment and electric power of themaximum path is larger than electric power of other paths (see FIG. 13).In other words, the head of the demodulation reception window isdeviated by an amount corresponding to the delay time of the maximumpath from ½ of the CP length in the forward direction. Also in thiscase, a larger amount of the output IFFT is rotated in accordance with adifference between the head of the demodulation reception window and theposition of ½ of the CP length. Here, the above-mentioned openenvironment refers to an open environment having no obstruction betweenthe base station and the user equipment and in which the base stationand the user equipment can be directly seen from each other, and isgenerally referred to as a line of sight (LOS). Further, examples of thearea mainly having an open environment include an environment having fewbuildings or the like, such as an rural area.

(6) A technology of determining the rotation amount of the IFFT outputin consideration of a variable margin of the reception window headposition in a counter device (i.e., a receiving device) will bedescribed below. In this case, the rotation amount of the IFFT outputmay be determined in consideration of an available variable margin ofthe reception window head position.

Specifically, when {CP length derived from methods of above-mentioned(2a) to (2c)}<{variable margin of reception window head position ofcounter device}, an amount of the IFFT output corresponding to ½ of theCP length is rotated. In contrast, when {CP length derived from methodsof above-mentioned (2a) to (2c)}>{variable margin of reception windowhead position of counter device}, an amount of the IFFT outputcorresponding to {variable margin of reception window head position ofcounter device}/2 is rotated.

As a value of the variable margin of the reception window head position,information notified of as user equipment capability or base stationcapability by using an RRC message may be used. Here, for example, theRRC message corresponds to RRC Connection Reconfig transmitted from thebase station to the user equipment to correspond to a Bearer Setuprequest from MME, or corresponds to RRC Connection Reconfig with which ahandover command is transmitted at the time of handover.

<1-3>

From the description above, for example, the first embodiment provides acommunication system including a base station device, and acommunication terminal device configured to perform radio communicationwith the base station device. The base station device and thecommunication terminal device when operating as a transmitting devicerotate inverse fast Fourier transform (IFFT) output, and copy a lastportion of the rotated IFFT output to a head of the rotated IFFT outputas a cyclic prefix (CP) to thereby generate a transmission signal sothat there is no phase rotation at a head of a demodulation receptionwindow set in a receiving device.

Further, for example, the first embodiment provides a configuration inwhich a rotation amount of the IFFT output may be in coordination withat least one of a CP length that is a length of the CP, a head positionof the demodulation reception window with respect to the CP, and avariable margin of the head position of the demodulation receptionwindow.

Second Embodiment

In the second embodiment, an OFDM transmission method involving aplurality of user equipments, e.g., control of the head position of thereception window, will be described.

The base station simultaneously communicates with a large number of userequipments, and therefore the scale of the device can be reduced iffunction units can be integrated. For example, a low-cost configurationcan be achieved if the function unit that performs OFDM demodulation(i.e., FFT) can be shared by all of the user equipments.

However, when every user equipment has a different head to be subjectedto FFT, individual timing control is required, which complicates thedevice configuration. Especially when an OFDM demodulating unit isprovided in a substrate different from a function unit that performschannel coding/decoding, control over a plurality of substrates isrequired, which complicates the control. For example, control inconsideration of an integrated circuit (IC) to be used, control delaydue to elongated wiring, a start-up procedure and an exceptionalprocedure due to insertion and extraction of a substrate,synchronization processing of a plurality of substrates, etc. isrequired.

<2-1>

In view of this, the second embodiment provides a technology for solvingthe problems as described above. Specifically, when the demodulationreception window cannot be matched in all OFDM symbols in aconfiguration of dividing an OFDM sub-carrier with the CP lengthdifferent in each user equipment to use the OFDM sub-carrier, at leastone of the rotation amount of the IFFT output and the CP length iscontrolled to match phases in every frequency at the head of thedemodulation reception window in a unit of data such as a subframe or aslot. Control examples are illustrated below.

The base station determines which timing should be the head of thedemodulation reception window for each component carrier (CC) based on(a) a set of corresponding radio formats, (b) a corresponding cellradius, or (c) corresponding maximum transmission electric power of thebase station itself. The system parameters of the above-mentioned (a) to(c) are stored in the non-volatile memory of the base station, but maybe specified by the OAM server. Alternatively, (d) a head value itselfof the demodulation reception window may be specified directly by thesystem parameter.

Next, how to determine the head position will be described. When thecorresponding radio formats include various CP lengths, it suffices thatapproximately 1/2 of the shortest CP length be set as the head of thedemodulation reception window (see FIG. 14). Similarly to the firstembodiment, the head of the demodulation reception window may becontrolled in consideration of the distance between the base station andthe user equipment, the moving speed and the moving direction of theuser equipment, the channel state such as delay distribution, thecapability of the device, etc.

Further, when the corresponding cell radius is also taken intoconsideration, it is effective to determine in the following manner (seeFIG. 15 and FIG. 16). According to a calculation value obtained by{corresponding cell radius}/{speed of radio wave}/2, how much delay isgenerated when the radio wave is transmitted for a distancecorresponding to twice the cell radius can be calculated. In otherwords, the delay required when the base station transmits a signal, theuser equipment transmits a response signal in response to thetransmission signal of the base station, and the response signal of theuser equipment reaches the base station can be calculated from theformula above.

When the shortest CP length is represented by CPmin, the followingformula is obtained.

CPmin>{corresponding cell radius}/{speed of radio wave}/2

In this case, it suffices that the demodulation reception window be setto approximately ½ of an amount of {corresponding cell radius}/{speed ofradio wave}/2 (see FIG. 15). In contrast,

CPmin<{corresponding cell radius}/{speed of radio wave}/2

in this case, it suffices that the demodulation reception window be setto approximately ½ of CPmin (see FIG. 16).

The user equipment rotates the IFFT output such that phases in everyfrequency at the specified head position of the reception window matchby using the following PUSCH. A procedure for setting theabove-mentioned head position will be described with reference to FIG.17.

When the head position of the reception window is set by using PUCCH (orPUSCH) being a control signal of L1 as illustrated in FIG. 17, if thebase station is provided with a function of measuring a change in apropagation environment of a multipath, a function of measuring a changein the distance between the user equipment and the base station, or thelike, such changes can be promptly handled.

When data to be transmitted to the base station occurs in the userequipment, the user equipment transmits PUCCH including a schedulingrequest (SR) in Step ST921. In Step ST922, the base station generatesinformation (which may be hereinafter referred to as head positionspecifying information) of specifying the head position of the receptionwindow in consideration of the radio formats of all of the userequipments made to transmit PUSCH in this case, such that the headpositions of the reception windows of PUSCH in this case match. Here,the head position specifying information may be generated inconsideration of the distance between the base station and each userequipment, the channel state between the base station and each userequipment, the moving speed and the moving direction of each userequipment, or the capability of each user equipment, instead of theabove-mentioned radio formats of all of the user equipments.

Note that the head position of the reception window may be different foreach set of the transmitting user equipments. Further, whencommunication is performed only with the user equipments close to thebase station, it suffices that the CP length be short.

Next, in Step ST923, the base station transmits a scheduling grant (SG)signal for notifying of data transmission grant to the user equipments.In this case, it suffices that the head position specifying informationof the reception window be added to the SG signal.

Consequently, the head positions of the reception windows of all of theuser equipments match. Particularly, the head positions of the receptionwindows of all of the user equipments match in a unit of data such as asubframe or a slot. Therefore, when the reference signal such as DMRSand the control information such as PUCCH are provided at the head of aunit of data, processing can be performed from the head without storinga plurality of symbols, and thus this is effective. Refer to FIG. 18.

Further, when the CP length is also transmitted with SG, ifcommunication is performed only with the user equipments close to thebase station, the CP length can be reduced as well. Therefore, overheadcan be reduced, and thus this is effective. Specifically, also when theIFFT output is not rotated and the head position specifying informationof the reception window is not transmitted, the head positions of thereception windows can be matched by transmitting only the CP length, andthus this is effective.

In contrast, when the reference signal such as DMRS and the controlinformation such as PUCCH are provided at a portion other than the headof the unit of data such as a subframe or a slot, as illustrated in FIG.19, it suffices that the head positions of the reception windows forthose signals be matched.

Even in a system in which an tolerable phase error is large and the IFFToutput rotation is not required, when the heads of the OFDM symbolsimmediately after CP are matched, the head positions of the receptionwindows match, and the processing is facilitated.

Here, it suffices that SR itself transmitted by the user equipment (seeStep ST921) be a unit that can have the same positions of the receptionwindows by sharing a time slot with SRs of other user equipments, forexample. SR is transmitted before adjustment with the base station isperformed (see Step ST921), and thus the IFFT output is rotated by anamount corresponding to a fixed value such as the same CP length or 1/2of the CP length, for example.

<2-2>

An example in which the user equipment performs communication withoutSR/SG will be described below with reference to FIG. 20.

In Step ST932, the base station performs grouping at a necessary timingfor each cell, each beam, or each CC depending on a communication stateof the user equipments served by the base station, or the distance orthe position with respect to the base station, to determine the CPlength and the IFFT output rotation amount. Then, in Step ST933, thebase station notifies the user equipments of the information determinedin Step ST932 by using broadcast information (an RRC message) or PDCCHof each group. In Step ST934, the user equipments rotate the IFFT outputsuch that phases in every frequency at the head position of thereception window of the base station match in accordance with theinformation notified of from the base station, and perform transmission.In Step ST935, the base station monitors the communication state witheach user equipment, and determines whether a change of groups or achange of the CP length and the IFFT output rotation amount for eachgroup is required. When the base station determines that a change isrequired, the base station executes the above-mentioned Step ST932.

Examples of group sorting will be described below.

In the first example, downlink groups are sorted depending onpropagation delay time from the base station. Even when the rotation ofthe IFFT output is shared within a group, time equal to or longer thantwice the propagation delay time is allocated to the CP length.According to this, the CP length can be reduced to be smaller than usualin the user equipments close to the base station, and thus throughput isenhanced.

In the second example, groups are sorted depending on whether successivetransmission is performed. When data is successively transmitted, it isalso effective to broadcast (i) the CP length or the head positionspecifying information of the reception window to be transmitted in thefirst time, and (ii) the CP length or the head position specifyinginformation of the reception window to be transmitted in the second andsubsequent times together to differ groups. The CP length to betransmitted in the second and subsequent times is controlled to a CPlength corresponding to an amount in which a channel may be changedbased on the first reception result. In accordance with this, therotation amount of the IFFT output can be controlled. Consequently,overhead can be reduced.

In the third example, groups are sorted by first transmission andretransmission of HARQ. The CP length at the time of retransmission iscontrolled to a CP length corresponding to an amount in which a channelmay be changed based on a reception result of the first transmission. Inaccordance with this, the rotation amount of the IFFT output can becontrolled. Consequently, overhead can be reduced.

In the fourth example, groups are sorted depending on a change rate ofthe distance between the user equipments and the base station. The userequipments are sorted into some groups in view of the fact that the CPlength and the IFFT output rotation amount are increased and decreasedby an amount corresponding to a value obtained by {controllable cyclesuch as RRC/L1}×{speed at which user equipment moves away from basestation}/{speed of radio wave}, to reduce the CP length of the terminalsmoving at a low speed. Consequently, overhead can be reduced, andthroughput can be enhanced.

In the fifth example, groups are sorted depending on elapsed time sincethe last transmission, or a state corresponding to the elapsed time. TheCP length when the elapsed time is short (in an active-state) can becontrolled to a CP length corresponding to an amount in which a channelmay be changed, as compared to the CP length when elapsed time is long(in an inactive-state). In accordance with that, the rotation amount ofthe IFFT output can be controlled. Consequently, overhead can bereduced.

The group sorting of the above-mentioned second to fifth examples canalso be applied to uplink. In a case of uplink, it is more effectivewhen the TA control is taken into consideration. Specifically, when a TAcontrol cycle and accuracy of TA are taken into consideration,generation of a group having unnecessarily large CP length and IFFToutput rotation amount can be prevented.

<2-3>

From the description above, for example, the second embodiment providesa communication system including a base station device, and acommunication terminal device configured to perform radio communicationwith the base station device. The base station device and thecommunication terminal device when operating as a transmitting devicerotate inverse fast Fourier transform (IFFT) output, and copy a lastportion of the rotated IFFT output to a head of the rotated IFFT outputas a cyclic prefix (CP) to thereby generate a transmission signal sothat there is no phase rotation at a head of a demodulation receptionwindow set in a receiving device. Particularly, when the base stationdevice performs radio communication with a plurality of communicationterminal devices, the base station device determines at least one of arotation amount of the IFFT output and a CP length that is a length ofthe CP so that a head of the demodulation reception window for each ofthe plurality of communication terminal devices aligns in a defined unitof data concerning transmission data.

Further, for example, the second embodiment provides a configuration inwhich the base station device may sort the plurality of communicationterminal devices into groups in accordance with a predeterminedreference, and may determine at least one of the rotation amount of theIFFT output and the CP length for each of the groups.

Third Embodiment

In the third embodiment, an OFDM transmission method involving aplurality of user equipments, e.g., the use of a sounding referencesignal (SRS), will be described.

In the 5G system, dividing one component carrier (CC) into a pluralityof user equipments or a plurality of communication channels to use theCC through use of filtered-OFDM has been studied. In this case,transmission and reception are performed with various symbol rates, andthus when the uplink/downlink symbol number is dynamically changed,segmented positions do not match. Therefore, for example, when thetransmitter and the receiver are located very close to each other at thetime of transmitting an uplink sounding reference signal (SRS), SRStransmission time and data reception time may be the same timing. Insuch a case, a low noise amplifier (LNA) or the like may be broken dueto excessive input in the receiver.

In the third embodiment, a method of enabling setting of a position ofswitching between uplink/downlink when filtered-OFDM is applied evenwith the problems as described above, and an SRS transmission methodwill be described.

<3-1>

The first example will be described below. In the first example, aposition of stopping downlink transmission is specified as a symbolposition in a channel having the longest symbol length in downlink CC(i.e., a channel having the smallest OFDM sub-carrier interval).Information of such a stopping position is notified by using an RRCmessage. Here, for example, the RRC message is broadcast information, orcorresponds to RRC Connection Reconfig transmitted from the base stationto the user equipment to correspond to a Bearer Setup request from MME,or corresponds to RRC Connection Reconfig with which a handover commandis transmitted at the time of handover. In the signal of SRS, similarlyto the existing 3GPP, randomization using a cell ID or the like as a keyis performed so as to recognize to which cell the signal has beentransmitted. Further, information about which user equipment transmits,information about a transmission timing, and information about atransmission frequency (a position of a sub-carrier) are specifiedtogether with the above-mentioned RRC, or by RRC Connection Reconfig forstarting sounding.

As for a channel other than the channel having the longest symbol length(i.e., the channel having the smallest OFDM sub-carrier interval), itsuffices that SRS be used in all symbols having a portion overlappingwith the channel having the longest symbol length. Further, it sufficesthat a state of not performing transmission, i.e., a null state, beapplied to a portion not overlapping with the channel having the longestsymbol length of the symbols having a portion overlapping with thechannel having the longest symbol length. Consequently, excessive inputto another user equipment can be prevented.

Further, in this case, transmission may be stopped during one symbol.

Further, a GI section without uplink and downlink transmission may belonger than the OFDM symbol. Energy of SRS is increased, and estimationwith good accuracy is enabled.

<3-2>

Although the description above illustrates an example in which theposition of stopping downlink transmission is specified as a symbolposition in a channel having the longest symbol length in downlink CC(i.e., a channel having the smallest OFDM sub-carrier interval), it isalso effective to specify the position of stopping downlink transmissionas a symbol position of a channel assigned at a center of CC, a channelfor synchronization, or a broadcast channel.

Such a specifying method will be described as the second example withreference to FIG. 21. In filtered-OFDM, OFDM signals having differentsymbol rates are transmitted and received with the same CC. In thiscase, signals of a channel assigned at the center of CC can be separatedby a low pass filter (LPF), whereas signals of other channels arerequired to be separated by a band pass filter (BPF). Specifically, thechannel at the CC center can be easily separated. Therefore, the channelat the CC center is suitable for monitoring on a regular basis even in astate out of data communication (idle or in-active state). Therefore,when the position of stopping downlink transmission is determined withreference to the symbol length at the CC center, control with goodaccuracy is enabled.

In FIG. 21, a channel 1 is assigned to a CC center frequency. The basestation determines a cycle and a timing for not performing transmissionbased on the number of user equipments served by the base station, acommunication state (e.g., uplink average data throughput), etc. FIG. 21illustrates an example in which the 6th and 7th OFDM symbols in thechannel 1 (see the symbols enclosed by the bold line) are determined astransmission stop time. Further, in other channels 2 and 3, transmissionof all of the OFDM symbols temporally overlapping with the 6th and 7thOFDM symbols of the channel 1 (see the symbols enclosed by the boldline) is stopped.

Meanwhile, the user equipments served by the base station are notifiedof information about at which timing and cycle transmission is enabledby using an RRC message, e.g., broadcast information. Instead of abroadcast channel, a signal corresponding to RRC Connection Reconfigtransmitted from the base station to the user equipment to correspond toa Bearer Setup request from MME, or a signal corresponding to RRCConnection Reconfig with which a handover command is transmitted at thetime of handover may be used.

Alternatively, several candidates concerning at which timing and cycletransmission is enabled may be notified in advance, and which candidatewill be used may be notified later. Specifically, the base station listsup the above-mentioned candidates, and notifies information of thecandidates by using the above-mentioned RRC message in advance. Afterthat, the base station notifies which format of the candidate will beused every time by using an L1 control information signal such as PDCCHassigned at the head (which may be at the second when there is DMRS atthe head) of the OFDM symbol.

FIG. 22 illustrates an example in which the user equipment transmits asounding reference signal (SRS) in a non-transmission period of FIG. 21.A period of SRS may be set to an integer multiple of an OFDM symbollength, and a remaining period may be set to a period in whichtransmission and reception are not performed as a gap interval (GI).

<3-3>

As illustrated in the example described above, SRS does not necessarilyrequire one symbol length. In view of this, it is also effective tospecify the position of stopping downlink transmission by using amultiple of a minimum unit time for transmitting SRS, instead ofstopping a downlink signal at a symbol position of a channel having thelongest downlink CC symbol length (i.e., a channel having the smallestOFDM sub-carrier interval) or a symbol position of a channel assigned atthe center of CC. Particularly, it suffices that {uplink and downlinkswitching time}+{minimum unit time of uplink transmission}×n (nrepresenting an integer) be specified as the stopping position ofdownlink transmission.

This example is effective when an uplink data amount is small, anddownlink throughput is intended to be increased.

The uplink minimum unit time is determined by a trade-off betweenthroughput improvement achieved by flexibility of control and complexityof control due to increase in a control information amount. For example,FIG. 23 illustrates an example that can correspond to 1/2 of the OFDMsymbol length assigned at a center frequency of CC, and this can improvethroughput.

The stopping position of downlink transmission may be specified by usingRRC similarly to the above.

<3-4>

From the description above, for example, the third embodiment provides acommunication system including a base station device, and a plurality ofcommunication terminal devices configured to perform radio communicationwith the base station device. The base station device divides onecomponent carrier (CC) into the plurality of user equipments or aplurality of communication channels to use the CC, and stops downlinktransmission at at least one symbol position of a channel having alongest symbol length in downlink CC, a channel assigned at a center ofthe CC, a channel for synchronization, and a broadcast channel.

Further, for example, the third embodiment provides a configuration inwhich the plurality of communication terminal devices may transmit asounding reference signal (SRS) while the downlink communication stops.

Modification

The embodiments and the modifications are merely illustrations of thepresent invention, and can be freely combined within the scope of thepresent invention. Any constituent elements of the embodiments and themodifications can be appropriately modified or omitted.

While the invention has been described in detail, the foregoingdescription is in all aspects illustrative and not restrictive. It istherefore understood that numerous modifications and variations can bedevised without departing from the scope of the invention.

EXPLANATION OF REFERENCE SIGNS

-   -   200 communication system, 202 communication terminal device, 203        base station device

1. A communication system comprising: a base station device; and acommunication terminal device configured to perform radio communicationwith the base station device, wherein the base station device and thecommunication terminal device when operating as a transmitting devicerotate inverse fast Fourier transform (IFFT) output, and copy a lastportion of the rotated IFFT output to a head of the rotated IFFT outputas a cyclic prefix (CP) to thereby generate a transmission signal sothat there is no phase rotation at a head of a demodulation receptionwindow set in a receiving device.
 2. The communication system accordingto claim 1, wherein a rotation amount of the IFFT output is incoordination with at least one of a CP length that is a length of theCP, a head position of the demodulation reception window with respect tothe CP, and a variable margin of the head position of the demodulationreception window.
 3. The communication system according to claim 1,wherein when the base station device performs radio communication with aplurality of communication terminal devices, the base station devicedetermines at least one of a rotation amount of the IFFT output and a CPlength that is a length of the CP so that a head of the demodulationreception window for each of the plurality of communication terminaldevices aligns in a defined unit of data concerning transmission data.4. The communication system according to claim 3, wherein the basestation device sorts the plurality of communication terminal devicesinto groups in accordance with a predetermined reference, and determinesat least one of the rotation amount of the IFFT output and the CP lengthfor each of the groups.